This invention relates to a method and apparatus for removing pressure tubes from nuclear reactors. In particular, the invention permits a pressure tube to be removed from the calandria of a nuclear reactor in a manner which requires the removal of only one end fitting from the reactor end shield.
In CANDU type nuclear reactors, nuclear fuel is contained in pressure tubes. Each pressure tube is concentrically located within a horizontally disposed calandria tube. Heavy water moderator surrounds the calandria tubes and heavy water coolant circulates through the pressure tubes around the fuel elements.
Each pressure tube extends horizontally through the calandria between the reactor end shields. Each end of the pressure tube is connected to an end fitting which extends through the end shield. The pressure tube is joined to the end fitting by means of a roll joint.
There is a requirement in nuclear reactors to periodically replace pressure tubes, for example in the event that an individual pressure tube develops a leak. The current practice for removal of a pressure tube requires the removal of both end fittings from the end shields at each end of the reactor. To do this, a roller cutter is introduced into the inside diameter of the pressure tube through the end fitting and is advanced to approximately the centre of the tube. The tube is then cut into two pieces. Second and third cuts are made through each pressure tube half, close to each end fitting. The two severed lengths of pressure tube are temporarily retained in the calandria tube. Both end fittings with the stub of pressure tube attached thereto are withdrawn from the end shields. The stub portion of the pressure tube is highly activated and accordingly both end fittings with attached stubs are discarded into separate flasks. The severed lengths of pressure tube are then discarded into a receiving flask.
The requirement to remove both end fittings from the reactor significantly increases the time and cost to remove pressure tubes and increases the radiation dose levels of the fuel channel replacement staff. Thus there is a need for a pressure tube removal procedure which facilitates removal of pressure tubes from nuclear reactors and reduces the amount of time required to complete the procedure.
In accordance with the present invention, there is provided a method and apparatus for removing a pressure tube from a nuclear reactor which requires removing only one end fitting from the reactor plate, and leaving the opposite end fitting intact.
Thus, in accordance with the present invention, there is provided a tool set for disengaging an interference fit between a concentrically aligned outer tube and an inner tube comprising a milling tool adapted to be inserted into said inner tube and comprising a cutting head for forming in the inside surface of the wall of said inner tube adjacent said interference fit an area of reduced wall thickness; a collapsing tool adapted to be inserted into said inner tube and comprising a pair or opposed jaw members for engaging the inside surface of the wall of said inner tube on either side of said area of reduced thickness and a means for activating said jaw members to draw said engaged area toward each other thereby reducing the diameter of said inner tube and disengage said inner tube from said outer tubes.
In accordance with another aspect of the present invention there is provided a tool for milling the inside surface of the wall of a tubular member at a position intermediate the ends of said tubular member, said tool comprising a tubular mounting sleeve adapted to be closely received in and fixed to one end of said tubular member; an elongated cylindrical body having a proximal end and distal end, said proximal end disposed within said mounting sleeve for longitudinal and rotational movement with respect thereto; positioning means fixed to said mounting sleeve for adjusting the longitudinal and rotational positions of said elongated cylindrical body with respect to said mounting sleeve; centering mechanism located within said cylindrical body at said distal end for positioning said body in spaced relation within said tubular member; a milling head located within said cylindrical body intermediate said proximal and distal ends, said milling head comprising a cutting member adapted to be moved from a first position within said cylindrical body and a second position projecting through an opening in said cylindrical body for engagement with the inside surface of the wall of said tubular member; and depth of cut control means located at said proximal end for controlling movement of said cutting member between said first and second positions.
In accordance with another aspect of the present invention, there is provided a collapsing tool for disengaging an interference fit between an outer tube and an inner tube, said inner tube having a weakened area of reduced wall thickness formed on its inside surface adjacent the area of said interference fit, said tool comprising a tubular mounting sleeve adapted to be closely received in and fixed to one end of said tubular member; an elongated cylindrical body having a proximal end and distal end, said proximal end disposed within said mounting sleeve for longitudinal and rotational movement with respect thereto; positioning means fixed to said mounting sleeve for adjusting the longitudinal and rotational positions of said elongated cylindrical body with respect to said mounting sleeve; a jaw assembly located inside said elongated body at said distal end, said jaw assembly comprising a pair of opposed jaw members projecting substantially radially through an opening in the wall of said body, said jaw members capable of being moved from a first open position for engaging the inside surface of the wall of said inner tube on either side of said weakened area to a second closed position to draw said engaged areas toward each other thereby reducing the diameter of said inner tube section and disengaging said inner tube from said outer tube; control means located at said proximal end for controlling the movement of said jaw members between said first and second positions.
In accordance with another aspect of the present invention, there is provided a method for disengaging an interference fit between an outer tube and an inner tube comprising forming in the inside surface of the wall of said inner tube adjacent said interference fit a longitudinal central weakened area of reduced wall thickness; forming in the inside surface of the wall of said inner tube on each side of said central weakened area a groove having longitudinal abutment edges; engaging the abutment edges of said side grooves with opposed jaw members and activating said jaw members to draw said engaged abutment edges toward each other, thereby reducing the diameter of said inner tube section and disengaging said inner tube from said outer tube.